Getting True Battery Voltage

Scenario: asking for a friend

Suppose you were using a heavy load on your battery such as a winch for recovery for 20 minutes. It was grueling recovery with several pulls, but you made it out in 1 piece. Then, say you read 13.8 volts on your scan gauge for example and shut the truck down for a break.

What I want to know, is there a way to read true battery voltage regardless of the voltage on the whole system while the engine is running? If this scenario played out, would you need a jump after shutting down the engine because the true voltage was lower than 13.8 because your reading charge from the alternator too?

Silly I know, but I cannot get a straight answer from people.

 

You can only measure the battery voltage if the engine is off. If the engine is running you are measuring the alternator voltage. I don’t get why you want to measure battery voltage while the engine is running. It doesn’t matter what the battery voltage is while the engine is running. Keep the engine running while winching and keep it running after to replenish your battery.
Also normal battery voltage for a fully charged battery is around 12.6 volts. You shouldn’t be getting 13.8 with the engine off.

 

I realize this.

It seems like a stupid question, but I was curious if there was a way to check actual voltage while the alternator is supplying charge to the system. I suppose one would have to disconnect the alternator momentarily to get a more accurate reading of the battery's charge, which doesn't seem easy to do or safe.

(What I aim to do is similar in concept to how you can read your phone's battery level while charging simultaneously.)

This is why I posted in the technical section.

I have searched the web and other related forums and no one has targeted this as I am here.

Thanks to anyone interested in entertaining my madness

 

Best way to check the capacity of the battery while running would be to install an in-line shunt and measure the charging current.

 

A winch will draw hundreds of amps in a heavy pull for as long as you are running it. The alternator puts out 135 amps in my case. The difference current comes from the battery reserve. I used to plow snow with an old Meyers "Electro-lift" pump. The Jeep would be running, alternator fine, lights bright. After plowing a driveway that needed lots of lifts I stalled the Jeep in drift....battery dead because the alternator could not keep up with the draw.

If you have been making a long,heavy pull keep the truck running,during and afterward for at least 15-20 minutes. Install dual batteries if you need to using correct cable gage and/or carry a spare with jumper cables or jump pack. Turn off any un-needed electrical loads during the pull too. The running battery voltage will drop during a heavy pull, as it does during cranking, and can be monitored anywhere in the system , even the 12 volt outlet in the dash, and may seem ok... unloaded. It doesn't mean much though because the battery reserve will have been discharged to the point that the truck won't crank over,if stalled, until that discharge has been regained.

The charging circuit is monitored by the ecm and the alternator output is controlled by the ecm to produce the right charging rate for the battery in normal conditions. It cannot allow for hundreds of amps of recharge current though. Recharging takes time.

 

This is more of what I'm peering into on this topic. I want to watch actual voltage on the battery alone and regardless of what the whole system is experiencing as a result of the alternator charging.

="Troyken, post: 20989119, member: 151577"]A winch will draw hundreds of amps in a heavy pull for as long as you are running it. The alternator puts out 135 amps in my case. The difference current comes from the battery reserve. I used to plow snow with an old Meyers "Electro-lift" pump. The Jeep would be running, alternator fine, lights bright. After plowing a driveway that needed lots of lifts I stalled the Jeep in drift....battery dead because the alternator could not keep up with the draw.

If you have been making a long,heavy pull keep the truck running,during and afterward for at least 15-20 minutes. Install dual batteries if you need to using correct cable gage and/or carry a spare with jumper cables or jump pack. Turn off any un-needed electrical loads during the pull too. The running battery voltage will drop during a heavy pull, as it does during cranking, and can be monitored anywhere in the system , even the 12 volt outlet in the dash, and may seem ok... unloaded. It doesn't mean much though because the battery reserve will have been discharged to the point that the truck won't crank over,if stalled, until that discharge has been regained.

The charging circuit is monitored by the ecm and the alternator output is controlled by the ecm to produce the right charging rate for the battery in normal conditions. It cannot allow for hundreds of amps of recharge current though. Recharging takes time.[/QUOTE]

I can totally see this happening because the voltage read on the system while running is not the battery's. Excellent point to bring a jump pack or cables incase this happens, but like I was saying it would be nice to just see what the battery's charge is so you know WHEN the reserve has been regained.


Thanks so much for the replies!

 

I don't know what cleverness is employed to monitor or determine a cell phone battery remaining charge but I suspect monitoring the specific gravity of the electrolyte of a lead acid battery would give you the state of charge independent of voltage. Those values are and have been known for decades.

 

There is no lead acid in a lithium ion battery. As I've posted above, it's the same technology that's been around since the invention of DC power. State of charge is measured by charging current. Specific gravity is another method to determine charge but not the sole method. If the battery is not actively being charged the state of charge is determined by the OCV

 

I know about lithium ion batteries, but I said lead acid battery. The o/p is interested in the remaining state of charge of the battery I think. He doesn't seem to care about the charging current or voltage. An inline shunt allows you to measure the voltage across the known shunt resistance and that is proportional to the current. That is not much different than a series ammeter showing the charging current and assuming it drops to near zero (via regulator or ecm control) when fully charged or else you would boil the battery dry while the engine was running without much electrical load. I guess that would give you an idea of the state of charge if the charge current approaches zero when full. The specific gravity of the electrolyte will give you the state of charge and has been documented and is adjusted for temperature. It is an old way,certainly not the only way and the O/P is looking for some way to do it. Maybe he can figure out how to monitor that parameter. It is not possible to measure the open circuit voltage of the battery corresponding to the state of charge (12.6v being fully charged) without disconnecting it as far as I know. Not very convenient to do on a car and not a good idea to do at any time while running. I think manual marine dual battery switches are make before break for "battery 1,2 or both" positions for this reason. Boaters are cautioned not to turn off any battery switch before shutting the engine down.

My Lithium Ion cordless tool batteries show remaining charge. I don't know what method they use to determine it (probably simply voltage). Cell phones are done the same way via some type of charge controller.

Anyway, a battery, dead from winching, is still dead.... measured or not. Good luck with your project.

 

Okay, so... Could I use a voltmeter with a very low resistance to measure the voltage right off the battery terminals? Haha I feel like I've done this but maybe my experience was that the voltage was above 12.6 volts. Trying this again after work. Certainly in the right direction of what I'm trying to achieve so thanks a bunch

 

Op, the battery is tied directly to the alternator. Its not possible to check voltage separate from the alternator while the engine is running. If the reason you want to know this measurement is to tell if the battery is weak, check cranking voltage. If it drops below 9 volts you probably need a new battery

 

Even if you were to isolate the battery with a disconnect switch while the engine was running, the battery would hold a surface charge for a few minutes. No different than shutting off the truck to check voltage.

 

Yes, it’s based on voltage. My understanding when I asked this question to the EE’s I know, I was told that voltage is a good predictor of state of charge for lithium ion batteries. It’s gets tricky under heavy loads (or when there is long wires) due to the voltage drop to shut the tool off at the right time to protect the battery pack from undervoltage.

 

No. You will want to install an ammeter in line to measure the current from the alternator to the battery. That will consist of an in line shunt rated to pass the current of the alternator, and the taps for the meter wire

 

Just stick a multimeter on your battery with the truck off

 

I think shutting off the truck is the part his trying to avoid. In case the voltage is so low that it's insufficient to restart.

 

If the battery is actively being charged by the alternator there is no way I am aware of the measure the current charge of the battery itself. Even if you had a way to interrupt the electrical current from the alternator and measure the battery charge it would not be accurate as the battery always shows a slightly higher charge immediately after turning off the engine.
The question really has no practical relevance. How often is someone going to use their winch to the point that it kills the battery and then turn the vehicle off? Why would anyone turn the engine off after a long winch pull knowing that the engine may not start? After winching keep your damn vehicle running. There’s no reason to shut down the engine after a long winch pull. If you are a frequent off-roader who does a lot of winching you should travel with a second vehicle, never alone who could jump your vehicle and you should carry an extra battery.

 

Easy now. It was a technical question and it is valid. As 6 gearT444E stated, it is possible to measure the current being sent to the battery from the alternator using an in line ammeter. That way you can tell when the battery is charged enough based on the current drop. Then you can safely shutdown your engine with confidence instead of guessing. I appreciate all the responses. I will be testing this soon.

 

I wonder if the cheap obd Bluetooth scanners will show alternator duty cycle. Might give some insight into what the battery is up to.